The invention relates to an apparatus for detecting step-out of a suspension control system. A suspension which is mounted onboard a vehicle, for example, is provided with a shock absorber (or absorbers) including a mechanism for adjusting a damping factor and a controller which drives the mechanism to set the damping factor of the shock absorber at a target value in order to reduce the amplitude of the oscillation of a sprung weight (such as a car body) which is caused by the oscillation of an unsprung weight (such as wheels).
Various proposals for such controllers have been made in the prior art. For example, Japanese Laid-Open Patent Application No. 208,108/1990 discloses a damping control in a manner corresponding to a vertical, lateral or longitudinal acceleration as well as a damping control corresponding to a vehicle height (the height of car body--the height of wheel).
Japanese Laid-Open Patent Application No. 276,807/1991 discloses a damping control in which the vertical acceleration of the sprung weight is integrated to calculate a rate of change in the vertical direction of the spring weight while the displacement of the unsprung weight is differentiated to calculate a rate of change in the vertical direction of the unsprung weight, with a target value obtained as a required damping force which is calculated on the basis of the rates of change in the vertical direction of both the spring weight and the unsprung weight.
Also, Japanese Laid-Open Patent Application No. 276,808/1991 discloses a damping control in which the displacement of the unsprung weight is differentiated to calculate a rate of change in the vertical direction of the unsprung weight while a rate of change in the vertical direction of the spring system is estimated from the displacement of the unsprung weight, with a target value determined as a required damping force which is calculated on the basis of the rates of change in the vertical direction of both the sprung weight and the unsprung weight.
Furthermore, Japanese Laid-Open Patent Application No. 15,113/1992 discloses a damping control in which the vertical acceleration of the spring system is integrated to calculate a rate of change in the vertical direction of the sprung weight while a damping force corresponding to the ratio of the rate of change in the vertical direction to the vertical acceleration is calculated, thereby choosing it as a target value.
Finally, Japanese Laid-Open Patent Application No. 170,171/1988 discloses a steering control in which the presence or absence of a step-out of a stepping motor is detected from the comparison of an output of a sensor which detects the amount by which steerable road wheels are steered against its target value.
In a skyhook control of a suspension used in an automobile, for example, a damping factor adjusting mechanism of a shock absorber, which functions as an actuator, must be frequently driven. However, if a stepping motor is used to drive the actuator, such frequent drive may cause a step-out of the motor. Specifically, the stepping motor may fail to track a signal which is output from a controller, resulting in a deviation between a desired actuator position (damping factor) and an actual actuator position of the controller. The occurrence of such step-out fails to provide a normal damping control, resulting in a loss of the skyhook control.
Accordingly, in order to maintain the reliability of the suspension control system, it is important to detect the occurrence of a step-out of a stepping motor or the like. However, there have been no such detecting means in the conventional suspension control systems. Consequently, once the step-out occurs, a normal condition cannot be resumed unless the position of the actuator is initialized again.
In different fields, it is known to detect the occurrence of a step-out of a stepping motor by coupling a position encoder to the drive shaft of the stepping motor, and normally comparing the angular position detected by the encoder against a target position, thereby detecting the occurrence or not of the step-out. However, a position encoder of this kind is expensive and requires a fine adjustment of the mounting position. In addition, a suspension control system requires such a position encoder to be mounted for each of four shock absorbers, and accordingly the use of such position encoder is not practical.